1. Field of the Invention
The present invention relates to a digital image forming apparatus such as a digital printer or a digital copying machine which is capable of precisely expressing gradation by using light intensity modulation method.
2. Related Art
In an apparatus of forming an image by modulating light intensity, an exposure process is performed against a photoconductor at a light intensity corresponding to a read-out image density signal.
On forming a half-tone image, the influence on the gradation characteristic must be taken into account.
As a whole, the density of image read from a document is not proportional to the density of an image generated on the surface of the photoconductor depending on the photosensitive characteristic of photoconductor, the physical characteristic of the toners and the environmental conditions including temperature and humidity. This results in the generation of non-linear characteristic which deviates from the ideal proportional characteristic to be realized. Conventionally, this nonlinear characteristic is called "gamma characteristic" or "gradation characteristic", which substantially makes up a main cause of deterioration of the fidelity of a reproduced image obtained especially from a document containing a half-tone image.
Then, a digital image forming apparatus preliminarly corrects the output characteristic against the input data of an image of the density of the document, and irradiates a light beam against the photoconductor by applying a light intensity corresponding to the corrected image density data so that the proportional characteristic can eventually be realized. This is called the gradation correction. In other words, at low gradation levels, the digital image forming apparatus outputs a power of light-emitting surpassing the read-out image density data, while at high gradation levels, a less amount of power of light-emitting in order to adjust the density of a reproduced image to be proportional to the gradation level.
Nevertheless, the light intensity modulation method available today generates quite noticeable nonlinear gradation characteristic. The non-linear gradation characteristic is generated by the photosensitivity of photoconductor, physical properties of toners, the light-emitting condition and the like. The light intensity of laser beam available for performing the exposure process has a Gaussian distribution characteristic, and a laser beam has for example 45 micrometer (in the main scan direction).times.75 through 84 micrometer (in the subscan direction) of half width. On the other hand, a dot has a size of for example 63.5 micrometer (in the main scan direction, 400DPI).times.84.7 micrometer (in the sub-scan direction, 300DPI). When performing a scanning operation against the surface of the photoconductor, the laser continuously emits a beam onto the photoconductor during a period corresponding to the length of a dot. As a result, each dot is formed by applying a light intensity corresponding to the magnitude of the read-out image density signal. Nevertheless, the electric potential of an electrostatic latent image formed on the surface of the photoconductor is not uniformly distributed. Furthermore, as a result of the development processing, toner images of adjoining dots superimpose themselves.
In consequence, the greater the non-linearity of the gradation characteristic, the greater the amount of correction needed for properly adjusting the gradation of the read-out image density signal. When the gradation characteristic is affected by the scattering of characteristics or deterioration due to aging of components of the digital image forming apparatus, then, the precision of the gradation correction becomes worse and the gradation characteristic becomes unstable, thus significantly degrading the fidelity of a reproduced image. Accordingly, it is desired that the non-linearity of the gradation characteristic be minimized as small as possible.